The group biography is a daunting task. Chronicling the life of a single subject is an exhausting feat of research, tact and elision. Bundling half a dozen lives in some factitious intertwining is asking for chaos. Only the freakishly energetic Humphrey Carpenter ever seemed to relish the experience, as he charted the heyday of Waugh, Powell, Betjeman and co in The Brideshead Generation.

Now Jeremy Lewis – top-notch biographer of Cyril Connolly, Tobias Smollett and the Penguin founder Allen Lane – takes on the lives of Graham Greene's siblings and cousins, plus a squad of charismatic figures from the 1930s and 1940s. It's a massive undertaking, a fluent socio-political history of the British intelligentsia in the 20th century's most turbulent years.

How pleasing to find that the upper-middle-class Greenes owed their fortune to Abbot Ale. Their ancestor Benjamin founded what became the Greene King brewery in Bury St Edmunds in the 1790s. Graham and his generation were the offspring of two brothers. Charles was a high-minded academic who became headmaster of Berkhamsted School, and married the tall, beautiful and coldly aloof Marion, who bore six children: Molly, Herbert, Raymond, Graham, Hugh and Elizabeth.

Charles's brother Edward ("Eppy") was a more impulsive, entrepreneurial chap, who worked for a coffee merchant in Brazil, made a fortune, married a German girl called Eva and spawned another sextet: Ben, Eve junior (known as "Ave"), Edward (known as "Tooter"), Barbara, Felix and Katharine. They and their parents left Brazil for England in 1910 and took over a 17-bedroom Hall at the other end of Berkhamsted. The family was thus divided into the poor-but-clever "School House Greenes" and the rich-but-dreamy "Hall Greenes," with servants but little sense.

Of the 12 cousins, Lewis concentrates on six who led vivid, if hectic, public lives (the girls don't get much of a look-in). Graham, the most famous, was influenced from boyhood by Victorian and Edwardian detective stories and tales of derring-do, and grew up to write Brighton Rock, The Heart of the Matter and The Third Man. He also worked for the British secret service in Cairo, after being enlisted into MI6 by his younger sister, Elisabeth. He formed an initially sexless mariage blanc with Vivien Dayrell-Browning, with whom he corresponded in emetic baby-talk, and sought relief with prostitutes.

His brother Raymond was a physician with a flair for mountaineering. He joined the unsuccessful 1933 attempt on Everest as chief medical officer and authority on the effects of altitude and oxygen deprivation; but when a combination of rubbishy equipment, terrible food, illness and sulks scuppered the expedition, Raymond's most useful virtue turned out to be his skill as a soothing raconteur. Hugh was precocious - at four, he ordered the novelist Mrs Humphrey Ward to stop sitting in his mother's chair - and became a brilliant journalist. From Berlin he covered the rise of the Nazis for the Daily Telegraph. One article, in which he argued that Hitler's territorial ambitions wouldn't stop with the Rhineland, was dropped after the Telegraph's proprietor, Lord Camrose, was assured by Von Ribbentrop that there wasn't a word of truth in it.

Hugh was banished from Germany, allegedly for laughing at the spectacle of the tiny Josef Goebbels being manhandled onto a train by a burly SS man. Later he became director-general of the BBC, helmsman of That Was the Week, That Was and scourge of Mrs Mary Whitehouse.

Felix was a feckless idealist and restless traveller. He dropped out of Cambridge to see the world, starting with mid-Depression America, visiting Hollywood and General Motors, working as a fruit picker in California and an ad man in New York. In the 1930s he arranged for exchange trips for the Hitler Youth and boy scouts in Berkhamsted. He set up the BBC's foreign broadcast station in Canada, made celebrated documentary films, joined Aldous Huxley's proto-hippie commune in California (where he imported antiques and cashmere jumpers) and wrote two books in praise of Communist China.

Ben, a hulking dreamer of six-feet-eight, spent his sorry career lurching from one ill-considered do-gooding scheme to another. Instinctively socialist, a Labour party activist and committed pacifist who foraged for sugar and blankets for starving refugees and persuaded the British authorities to admit more Jewish children than they planned, Ben nonetheless ended up joining the pro-Nazi, fascistic British Council for a Christian Settlement in Europe, whose members were duly rounded up by the 1940 Emergency Powers Act ("Collar the lot!" demanded Winston Churchill). He found himself interned alongside Oswald Mosley.

Then there was Herbert, eldest of the School House Greenes. After the others' conspicuous achievements, Herbert is rather a relief. A classic black sheep, drunkard and loser, he lived off his parents and brothers, trying careers and failing. He embodied that very English phrase "remittance man", one who lives abroad on family handouts. He was packed off to Brazil but drank himself into penury and was packed home, he failed as a tobacco farmer in Rhodesia and, while living on the sponge in London, decided to try being a spy – first for the Japanese, then the Russians.

His application letter to the Russian embassy was forwarded to MI5, who became understandably sick of the amateur spook. He was later found driving a relief van in the Spanish Civil War, and embezzling funds for charity. "I wish to goodness he'd shoot himself" said an exasperated Graham, who put him in the novel England Made Me as the seedy conman, Anthony Farrant.

Lewis chronicles their vivid stories mostly from the 1920s to the 1940s, "when my subjects were full of hope and promise," he says in a Preamble, "and their lives more interlaced than they later became." He highlights several concerns common to inter-war English family members: their readiness to travel to un-luxurious places in search of novelty and excitement; the political conscience many displayed when confronted by deprived workers; their readiness to embrace a little espionage, complete with mackintoshes and abbreviated codenames; and the network of influence they all enjoyed, finding each other jobs from Fleet Street to the Foreign Office, putting in a good word, guaranteeing each other's bona fides.

Unfortunately, it's hard to warm to the family. The men sound an appalling bunch. Amid his fellow-correspondents Hugh was considered chilly and restrained, neither clubbable nor a late-night drinker. "He neither considered other people," said his future wife, Helga Guinness, "nor did he need other people much." He treated his children as if they were "nice nephews".

Graham, when his marriage to the baby-talking Vivien was finally consummated, also abhorred children, while his sister Barbara (who accompanied him on his trip to Liberia, and wrote a much better book than Journey Without Maps) confessed to being alarmed by his brain, so "sharp and clear and cruel".

The Hall Greenes, by contrast, were emotionally unfocused. Ben and Felix spent their lives as unfulfilled drifters and social utopians. Maybe their German mother's influence caused them to praise aspects of Nazi policy: sometimes it sounds more like autism. Felix's sexual dyslexia caused constant problems. Handsome and charming, he drove a succession of women into paroxysms of adoration, soon followed by disappointment.

Among his conquests was the painter Georgia O'Keeffe, who fell in love with the cerulean-eyed limey. "We've been fond of each other, but you haven't ever really given me anything," she told Felix when he visited her in New Mexico. "Nothing has come back from you. I think you'd better go."

Perhaps the Greenes' restless travelling, adventuring, politicking and spying was to make up for a central family blankness. Lewis's hugely detailed, exhaustively researched family saga makes you marvel, again and again, at the strangeness of this very English family, while leaving you fervently grateful you're not actually related to them.

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